2012 security operation will vet 500,000 people before Games

Flaming June: The Olympic torch pauses in Liverpool during its long journey around the United Kingdom before the Games begin in London next month (PA)

The Home Office is in the process of vetting half a million people in an attempt to ensure there will be no breaches of security from anyone involved in next month's London Olympics. All employees, volunteers, competitors, officials and media from over 200 countries attending the Games will be screened in the biggest such operation since the Second World War.

A process that begun last October is now nearly two-thirds complete with only around 100 applications so far said to have been refused. Some of those have reportedly been on the advice of MI5, although it is also claimed that there is no evidence of any attempts by terrorist groups to infiltrate the Olympic workforce.

Among those remaining to be scrutinised are the extra guards G4S, the private security firm that won the contract to staff the Games, had to recruit after the numbers required around the Olympic Park were hopelessly underestimated by Locog, the organising committee.

From an initial estimate of 2,000 the number rose to 10,000. Troops will also be deployed at the Park in Stratford. Security costs for the Games have spiralled to around £1bn, with £553m spent on in-venue security.

Among those vetted will be 10,500 athletes and the 70,000 Britons who have volunteered. A Home Office spokesperson said: "We are undertaking stringent checks on all those seeking accreditation. This rigorous process has been designed to ensure those working at the Games are fit to do so. We will leave nothing to chance in our aim to deliver a safe and secure Games that London, the UK and the whole world can enjoy."

The issue of whether to allow members of the Syrian national Olympic committee with ties to the country's regime to attend the Games has yet to be settled. Mofwaq Joumaa, president of the Syrian NOC, is a general in the Syrian army. If there is independent verification of human rights abuses in Syria then he will almost certainly be barred from coming to the UK. Anyone with a EU travel ban are not permitted to attend the Games, that includes Robert Mugabe and President Assad.

Security has become one of the most contentious issues around the Games from the dramatic late rise in costs to the deployment of surface-to-air missiles on top of residential tower blocks in east London.

Tight security yesterday surrounded the Olympic torch relay in Northern Ireland. Angus Keys, 12, pictured, from Co Fermanagh, carried the torch from Enniskillen Castle. Today it will cross into the Republic where it will be paraded in Dublin. Singers Jedward will be among the bearers.